Make note file extension configurable [v2.1] - SublimeNotebook

New version 2.1 released

Changelog

Added a new feature to allow users to set the file extensions that can be accepted as a SublimeNotebook "note". In v2.0 and before, only txt and md files were accepted as note files.
With this version, user will be able to add more formats and remove existing formats using a setting in settings.json.

The concerned settings.json setting looks as follows -

{
    "note_extensions": [
        "txt",
        "rst"
    ],
]

This means that only txt and rst files are treated as "note" by SublimeNotebook.

Implementation Details

Implementing this feature basically required changing the default settings.json structure here and then using the corresponding setting in the get_file_list method. The code change required to implement this feature can be seen in the commit fce50fb. Apart from that, I also updated the documentation, diff of which can be seen in the PR.